1. Field of the Invention
The present invention relates to a catalyst, which can efficiently purify soluble organic fractions (i.e., SOF) and soot-like components in an exhaust gas, being emitted from a diesel engine (hereinafter abbreviated to “DE ”), and which can reduce the emission of particulate materials.
2. Description of Related Art
Concerning gasoline engines, the harmful materials in the exhaust gases, which are emitted therefrom, have been reduced steadily because of the severe regulations on the exhaust gases and the developments of the techniques for coping with the regulations. However, concerning DE's, the regulations and the developments of the techniques are behind those of the gasoline engines because of the singular circumstance that the harmful components are mainly emitted as particulate materials. Accordingly, it has been desired to develop an exhaust gas purifying catalyst, which can securely purify the harmful components.
The purifying apparatuses for exhaust gases emitted from DE's, which have been developed so far, can be roughly divided into two categories. For example, there have been known an apparatus, to which a method using a trap is applied, and an apparatus, to which an open type SOF decomposition catalyst is applied. The former apparatus can be further divided into two categories, i.e., an apparatus without using a catalyst and an apparatus provided with a catalyst. Among them, the apparatus, to which a method using a filter is applied, captures the particulate materials by the filter so as to suppress the emission, and is especially effective to exhaust gases whose dry soot contents are high. However, there remain many problems to be solved in the apparatus, to which a method using a filter is applied, when the apparatus is put into actual applications. For instance, the apparatus requires a recycling apparatus for burning the captured particulate materials; when the captured particulate materials are burned, the filter may suffer from cracks in recycling; the filter might be clogged up by ashes; the apparatus might be complicated in terms the system, and so on.
On the other hand, in the apparatus to which an open type SOF decomposition catalyst is applied, a catalyst is utilized in which a catalytic ingredient, such as a platinum group element, is loaded on a loading layer, such as an activated alumina, similarly to a catalyst for purifying an exhaust gas, which is emitted from a gasoline engine, as disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 1-171,626, for example. The catalyst oxidizes and decomposes SOF together with CO and HC to purify them. The open type SOF decomposition catalyst has a drawback in that it exhibits a low reduction efficiency to dry soot. However, it is possible to reduce the content of dry soot by improving DE's and fuels themselves. Moreover, the apparatus has a great merit in that it does not require a recycling apparatus. Accordingly, furthermore technical improvements are expected to develop in the apparatus, to which an open type SOF decomposition catalyst is applied, in the future.
However, an open type SOF decomposition catalyst has a disadvantage in that the activities of the catalytic ingredient might be so low under a low temperature condition that the conversion of SOF lowers, though it can efficiently decompose SOF under an elevated temperature condition. Consequently, when the temperature of an exhaust gas is low as in the period of starting a DE, idling a DE, or the like, a phenomenon might occur in which un-decomposed SOF turn into soot and deposit in the honeycomb-shaped passages of the open type SOF decomposition catalyst. Then, a drawback might arise in that the catalyst is clogged up by the deposited soot so that the catalytic performance degrades.
Moreover, in an open type SOF decomposition catalyst, there might be a problem in that even SO2, which is contained in an exhaust gas, is oxidized to generate SO3 and SO4 in a high temperature region, and that the resulting SO3 and SO4 are turned into sulfates so that the emission of particulate materials increases inversely. This results from the fact that SO2 is not measured as particulate materials but SO3 and SO4 are measured as particulate materials because they are turned into and are emitted as sulfates. In particular, in exhaust gases, which are emitted from DE's, the oxidation reaction of SO2 is likely to take place, because an oxygen gas is present in a large amount therein.
In addition, in an open type SOF decomposition catalyst, it has been known that the catalytic ingredient is subjected to poisoning by sulfur, which is contained in exhaust gases, being emitted from DE's, in a large amount, so that the catalytic activities of the catalytic ingredient lower. Namely, SO2, which derives from sulfur in fuels, reacts with alumina in the catalytic loading layer to generate aluminum sulfate (Al2(SO4)3). Thus, the catalytic activities degrade, because the resulting aluminum sulfate covers the catalytic ingredient.
While, in the field of processing exhaust gases, which are emitted from boilers, a catalyst has been developed in which titania (TiO2), having a good sulfur poisoning resistance, is used as a catalytic loading layer and a catalytic ingredient, such as Pt, V, or the like, is loaded on the catalytic loading layer, and has been put into actual applications. However, such a catalyst does not exhibit an adsorption ability to SOF, and accordingly allows to emit HC as well as SOF as they are in a low temperature region.
In Japanese Unexamined Patent Publication (KOKAI) No. 4-267,928, there is proposed a catalytic apparatus for purifying an exhaust gas, which is emitted from a DE. In the catalytic apparatus, a first catalyst, which has a coating layer, such as an activated alumina layer, a zeolite layer, or the like, exhibiting a high adsorption ability, and which does not have a catalytic ingredient, is disposed on an upstream side of an exhaust gas flow; and a second catalyst, which has a second coating layer, such as a titania layer, a silica layer, or the like, exhibiting a low adsorption ability, and which has a catalytic ingredient loaded on the catalytic loading layer, is disposed on a downstream side of the exhaust gas flow.
In accordance with the catalytic apparatus, not only HC and SOF but also SO2 are adsorbed onto the upstream side first catalyst when the exhaust gas temperature is low, but SO2 is inhibited from being oxidized because the upstream side first catalyst does not have a catalytic ingredient. Then, in the downstream side second catalyst, HC and SOF, which are emitted from the upstream side first catalyst when the exhaust gas temperature is high, are oxidized and purified by the catalytic ingredient. While, SO2 is also emitted from the upstream side first catalyst, SO2 is inhibited from adsorbing onto and being oxidized by the downstream side second catalyst, because the downstream side second catalyst exhibits a low adsorption ability. Thus, the generation of sulfates is suppressed.
However, even the above-described catalytic apparatus for purifying an exhaust gas, which is emitted from a DE, is not satisfactory in terms of the purifying performance of SOF as well as the sulfur poisoning resistance. Consequently, it has been required to further improve the catalytic apparatus in terms of them.